The present invention generally relates to wireless communications systems and methods, and more particularly to wireless mobile terminals, systems and methods.
Public wireless radiotelephone systems are commonly employed to provide voice and data communications to subscribers. For example, analog cellular radiotelephone systems, such as designated AMPS, ETACS, NMT-450, and NMT-900, have long been deployed successfully throughout the world. Digital cellular radiotelephone systems such as those conforming to the North American standard IS-54 and the European standard GSM have been in service since the early 1990""s. More recently, a wide variety of wireless digital services broadly labeled as PCS (Personal Communications Services) have been introduced, including advanced digital cellular systems conforming to standards such as IS-136 and IS-95, lower-power systems such as DECT (Digital Enhanced Cordless Telephone) and data communications services such as CDPD (Cellular Digital Packet Data). These and other systems are described in The Mobile Communications Handbook, edited by Gibson and published by CRC Press (1996).
FIG. 1 illustrates a conventional terrestrial wireless communication system 20 that may implement any one of the aforementioned wireless communications standards. The wireless system may include one or more wireless mobile terminals 22 that communicate with a plurality of cells 24 served by base stations 26 and a Mobile Telephone Switching Office (MTSO) 28. Although only three cells 24 are shown in FIG. 1, a typical cellular radiotelephone network may comprise hundreds of cells, may include more than one MTSO 28 and may serve thousands of wireless mobile terminals 22.
The cells 24 generally serve as nodes in the communication system 20, from which links are established between wireless mobile terminals 22 and an MTSO 28, by way of the base stations 26 servicing the cells 24. Each cell 24 will have allocated to it one or more dedicated control channels and one or more traffic channels. The control channel is a dedicated channel used for transmitting cell identification and paging information. The traffic channels carry the voice and data information. Through the communication system 20, a duplex radio communication link 30 may be effected between two wireless mobile terminals 22 or between a wireless mobile terminal 22 and a landline telephone user 32 via a Public Switched Telephone Network (PSTN) 34. The base station 26 generally handles the radio communications between the base station 26 and the wireless mobile terminal 22. In this capacity, the base station 26 may function as a relay station for data and voice signals.
FIG. 2 illustrates a conventional celestial (satellite) communication system 120. The satellite wireless communication system 120 may be employed to perform similar functions to those performed by the conventional terrestrial wireless communication system 20 of FIG. 1. In particular, the celestial wireless communication system 120 typically includes one or more satellites 126 that serve as relays or transponders between one or more earth stations 127 and satellite wireless mobile terminals 122. The satellite 126 communicates with the satellite wireless mobile terminals 122 and earth stations 127 via duplex communication links 130. Each earth station 127 may in turn be connected to a PSTN 132, allowing communications between the wireless mobile terminals 122, and communications between the wireless mobile terminals 122 and conventional terrestrial wireless mobile terminals 22 (FIG. 1) or landline telephones 32 (FIG. 1).
The satellite wireless communication system 120 may utilize a single antenna beam covering the entire area served by the system, or as shown in FIG. 2, the satellite wireless communication system 120 may be designed such that it produces multiple, minimally-overlapping beams 134, each serving a distinct geographical coverage area 136 within the system""s service region. A satellite 126 and coverage area 136 may serve a function similar to that of a base station 26 and cell 24, respectively, of the terrestrial wireless communication system 20.
Thus, the satellite wireless communication system 120 may be employed to perform similar functions to those performed by conventional terrestrial wireless communication systems. In particular, a satellite radiotelephone communication system 120 may have particular application in areas where the population is sparsely distributed over a large geographic area or where rugged topography tends to make conventional landline telephone or terrestrial wireless infrastructure technically or economically impractical.
As the wireless communication industry continues to advance, other technologies will most likely be integrated within these communication systems in order to provide value-added services. One such technology being considered is a Global Positioning System (GPS). Therefore, it would be desirable to have a wireless mobile terminal with a GPS receiver integrated therein. It will be understood that the terms xe2x80x9cglobal positioning systemxe2x80x9d or xe2x80x9cGPSxe2x80x9d are used to identify any spaced-based system that measures positions on earth, including the GLONASS satellite navigation system.
A GPS system 300 is illustrated in FIG. 3. As is well known to those having skill in the art, xe2x80x9cGPSxe2x80x9d refers to a space-based trilateration system using satellites 302 and computers 308 to measure positions anywhere on the earth. The term GPS was originally introduced and often is used to refer to a system developed by the United States Department of Defense as a navigational system. However, for purposes of this application, the term GPS refers more generally to both the Department of Defense system and other space-based systems such as GLONASS. Compared to other land-based systems, GPS may be unlimited in its coverage, may provide continuous 24-hour coverage regardless of weather conditions, and may be highly accurate. While the GPS technology that provides the greatest level of accuracy has been retained by the government for military use, a less accurate Standard Positioning Service (SPS) has been made available for civilian use.
In operation, a constellation of 24 GPS satellites 302 orbiting the earth continually emit a GPS radio frequency signal 304 at a predetermined chip frequency. A GPS receiver 306, e.g., a hand-held radio receiver with a GPS processor, receives the radio signals from visible satellites and measures the time that the radio signals take to travel from the GPS satellites to the GPS receiver antenna. By multiplying the travel time by the speed of light, the GPS receiver can calculate a range for each satellite in view. From additional information provided in the radio signal from the satellites, including the satellite""s orbit and velocity and correlation to its onboard clock, the GPS processor can calculate the position of the GPS receiver through a process of trilateration.
More particularly, the GPS signal 304 generally consists of a spread-spectrum signal that has a code-length of 1023 chips (bits), and it is transmitted at a chip-rate of 1.023 MHz. This results in a code period of one millisecond. Overlaid on top of the spread-spectrum sequence is a 50 bits/second (bps) navigation message which, typically, contains ephemeris/almanac data as well as timing information which is used to timestamp the transmit time of the signal from the GPS satellite 302. The timestamp, which is generally transmitted in each sub-frame of the 1500 bit navigation message, each sub-frame consisting of 300 bits, is used to calculate the integer number of C/A (coarse acquisition Gold code) code lengths between the GPS satellite 302 and current position of GPS receiver 306.
Because of the slow transmission rate (50 bps) of the navigation message, it typically takes at least 6 seconds to decode a sub-frame of the navigation message. Because the start of decoding is asynchronous, the decoding of the timestamp could take up to twelve seconds. On average, it takes 9 seconds to decode a sub-frame and extract the timestamp in order to resolve the one millisecond ambiguity (from the code period).
The code-phase measurements for the GPS SPS signal have a one-millisecond ambiguity due to the inherent nature of the received GPS spread-spectrum signal. The spread-spectrum signal is generated by repeatedly transmitting a unique Gold-code for each of the GPS satellites. Each of the Gold-codes is 1023 chips (bits) in length, and they are transmitted at a 1.023 chips/sec rate. The chip length divided by the chip rate gives the Gold-code period of one millisecond. Measurement of the code phase determines the fractional number of Gold-code lengths between the GPS satellite and the GPS receiver. The problem remains of determining the integer number of code periods between the GPS satellite and the GPS receiver. This is typically resolved by decoding at least one sub-frame of the navigation message that is over-laid on top of the spread-spectrum signal using 50 bps BPSK modulation. Each sub-frame in the navigation message contains a time-stamp that represents the time of transmission of the sub-frame. The GPS receiver extracts the time-stamp from the naviagion message and subtracts it from the current time to estimate the integer number of code-periods from the GPS satellite and the GPS receiver. If the receiver estimate of the GPS time is in error the time-bias will be common to all of the pseudo-range measurements, and it may be removed when the point fix solution is calculated.
Before the data can be demodulated, the receiver generally needs to acquire the GPS signal 304 and achieve code and carrier lock. As well as increasing the time to obtain a GPS position fix, demodulating the GPS navigation data complicates the design of the GPS receiver 306.
It is known to combine a GPS receiver and radio communication receiver which receives information, such as GPS satellite Doppler information from a terrestrial base station, for use in determining position as described in U.S. Pat. No. 5,663,734 to Krasner entitled xe2x80x9cGPS Receiver and Method for Processing GPS Signalsxe2x80x9d. It is also known to provide a GPS receiver with GPS satellite position information transmitted over a communication channel supported by terrestrial cellular telephone or other radio packet data services as described in U.S. Pat. No. 5,365,450 to Schuchman entitled xe2x80x9cHybrid GPS/Data Line Unit for Rapid, Precise, and Robust Position Determinationxe2x80x9d. In addition, various proposals have been submitted to the T1 Standards Committee of the European Telecommunications Standard Institute (ETSI) regarding assisted GPS for GSM radiotelephone communication systems as described in the submittal entitled xe2x80x9cEvaluation Worksheet for Assisted GPSxe2x80x9d by Ericsson and SnapTrack submitted to the T1P1 working group (of the European Telecommications Standard Institute ETSI) on Jun. 3, 1998. Each of these approaches is generally directed to reducing the time required to determine a position from the GPS data and provide for the transfer of information about the GPS satellites to a combined mobile terminal/GPS receiver from communication networks, such as a terrestrial cellular network. However, these approaches typically require advantaged (clear) communication with the positioning/communication terminal and transfer of specific GPS satellite information to facilitate acquisition and calculation of position.
It is, therefore, an object of the present invention to resolve the problem of determining a position quickly and effectively from GPS satellite communications.
More particularly, it is an object of the present invention to resolve the one-millisecond ambiguity in GPS pseudo-range measurements.
It is a further object of the present invention to resolve the one-millisecond ambiguity in GPS pseudo-range measurements while reducing processing requirements which are required to be carried out by battery powered devices.
These and other objects are provided, according to the present invention, by providing methods and systems for determining a position of a mobile terminal that includes a satellite radiotelephone and a global positioning system (GPS) receiver based on knowledge of the position of the communication satellite spot-beam in which the mobile terminal is located. Using knowledge of the location of the communication satellite spot-beam, the present invention can resolve the one-millisecond ambiguity, that typically arises from the code repeat length where relative code phase may be determined but absolute code phase (i.e. one millisecond increments) could be off by one or more code periods. The spot-beam identifier is obtained from the satellite communication system and used to determine the geographic location of the spot-beam. Trial positions, i.e., candidate mobile terminal positions, are then selected to cover the spot-beam area, with spacing between trial positions provided to account for the one-millisecond ambiguity. Pseudo-range measurements are generated for each trial position and used to generate position fixes. The best position fix, for example a fix selection based on a consistency check, is then selected as the mobile terminal""s position. In an additional aspect of the present invention, the number of trial positions is reduced by selecting an arc of positions within the spot-beam as trial positions based on timing delay information from the satellite communication system.
In one embodiment of the present invention, a method is provided for determining a position of a mobile terminal. An identification of a spot-beam in which the mobile terminal is located is obtained from a satellite radiotelephone system communication and a plurality of trial positions within the spot-beam are generated. Candidate position fixes for the mobile terminal are generated for at least two of the plurality of trial positions based on GPS signals received by the mobile terminal from a plurality of global positioning system (GPS) satellites. One of the determined candidate position fixes is then selected as the position of the mobile terminal.
The candidate position fixes may be determined by calculating a plurality of initial pseudo-range estimates corresponding to the plurality of GPS satellites for one of the plurality of trial positions based on the GPS signals received by the mobile terminal, the GPS signals having a code length, including determining fractional code periods from the received GPS signals. The initial pseudo-range estimates may then be adjusted to produce a plurality of candidate pseudo-range estimates for the mobile terminal for the one of the plurality of trial positions and the candidate position fix for the mobile terminal may then be generated for the one of the plurality of trial positions based on the plurality of candidate pseudo-range estimates. The calculating, adjusting and generating operations may be repeated for each of the plurality of trial positions.
In one embodiment of the present invention, adjusting operations include truncating each of the initial pseudo-range estimates to produce a corresponding plurality of integer code length distances for the one of the plurality of trial positions and adding distances corresponding to the fractional code periods derived from the received GPS signals to the plurality of integer code length distances to provide a corresponding plurality of candidate pseudo-range estimates for the mobile terminal. Alternatively, adjusting operations may include modifying the plurality of initial pseudo-range estimates to account for relative differences between the fractional code periods derived from the received GPS signals to provide a corresponding plurality of candidate pseudo-range estimates for the mobile terminal. Range and position fix calculations may be carried out by the mobile terminal or at a remote location.
In another embodiment of the present invention, the calculating, adjusting and generating operations may be repeated for each of the plurality of trial positions to provide candidate position fixes for the mobile terminal for each of the plurality of trial positions. In another aspect of the present invention, the plurality of GPS satellites are first selected from visible GPS satellites based on the elevation of the visible GPS satellites relative to one of the plurality of trial positions.
In one embodiment of the present invention, the position fixes are least squares position fixes and at least four GPS satellites are used in determining position fixes. One of the candidate position fixes is then selected as the position of the mobile terminal based on a self-consistency check. The selection may be made using an overdetermined set of equations. In a particular embodiment, the selection is made by Q-R decomposing a direction cosine matrix determined from the candidate position fixes and multiplying non-constrained dimensions of a transpose of a Q matrix by delta pseudo-range values from the candidate position fixes.
In a further embodiment of the present invention, each of the trial positions is located no more than approximately 150 kilometers of another of the plurality of trial positions. The plurality of trial positions are generated on a grid covering the spot-beam. The spot-beam identifier is received from the satellite radiotelephone system communication and a geographic location of the spot-beam is calculated based on the received spot-beam identifier. The location calculation may be based on satellite ephemeris data obtained from the satellite radiotelephone system communication and antenna direction coordinates or, alternatively, on boundary points for the spot-beam stored in the mobile terminal.
In a further aspect of the present invention, satellite communication timing information is also obtained from the satellite radiotelephone system communication and the plurality of trial positions is generated based on the obtained timing information. The timing information may be timing delay information in which case the plurality of trial positions are selected along an arc of positions substantially equidistant from a satellite transmitting the satellite radiotelephone system communication based on the timing delay information. Depending on the accuracy of the timing delay information, the plurality of trial positions may be selected along at least one arc of positions substantially equidistant from a satellite transmitting the satellite radiotelephone system communication based on the timing delay information wherein each of the trial positions is located no more than approximately 150 kilometers of another of the plurality of trial positions.
In one embodiment of the present invention, the satellite communication timing information is requested from a satellite radiotelephone system satellite on a high margin random access channel. The satellite communication timing information is received from the satellite radiotelephone system satellite on a high margin paging channel. In an embodiment where the timing information is timing delay information, a request is transmitted which includes a timing update request having a request number identifier and a timing update is then received having an associated identifier corresponding to the request number identifier.
In another embodiment of the present invention, a GPS signal is received at the mobile terminal from each of the plurality of GPS satellites. A code phase is determined for each of the plurality of GPS satellites from the received GPS signals at the mobile terminal. The determined code phases are then provided to a remote GPS station from the mobile terminal which determines the candidate position fixes and selects one as the position of the mobile terminal.
While the present invention has generally been described above with reference to method aspects of the present invention, it is to be understood that the present invention also provides for mobile terminals and systems for determining a position of a mobile terminal.